VaButton and other components with background color should support gradient color

[asvae]

I'm really unsure whether we want this or not.

We discussed if we can...

[aluarius]

Discussed with Maxim. Not so difficult as at the first look but is still pretty complicated.

Some notes for myself:

• useTextColor (isTransparentBg) should rely only on opacity;
• rework forming of background color, opacity, mask and also text color & mask via before-after and CSS variables-classes. Ideally we want to remove js color transformations at all;
• check if everything is working well with gradient bg;
• update docs & demos.

[aluarius]

For buttons implemented here #2425

Not sure about other components... Let's discuss.